Mechanochemistry of zeolites: Part 2. Change in particulate properties of zeolites during ball milling

~

UTTEBWORTH I N E M A N N

Mechanochemistry of zeolites: Part 2. Change in particulate properties of zeolites during ball milling Cleo KosanoviE, Josip BroniE, Ankica Ci~mek, Boris SubotiE, and Ivan ~;mit

Ruder Bo~kovig Institute, P.O. Box 1016, 41001 Zagreb, Croatia Mirko Stubi~ar and Anton Tonejc

Department of Physics, Faculty of Science, University of Zagreb, 41000 Zagreb, Croatia Change of particulate and morphological properties of the samples obtained during the highenergy ball milling of zeolites A and X were studied by scanning-electron microscopy, X-ray diffraction, and determination of particle size distribution. It was found that decrease of crystallinity is followed by considerable change of particulate properties during the milling, but that the change of particulate properties and amorphization are two independent processes. Type of amorphization induced by ball milling was defined on the basis of the analysis of the change of effective crystallite size during the milling. KelnHords: Zeolites; amorphization; ball milling; particulate processes

INTRODUCTION Although the most pronounced effects of mechanical treatment, including ball milling, are lowering of particle size, increasing o f specific surface area, and change in particle morphology, use of high-energy ball milling process can also result in the production of composite metal powders with a fine microstructure and different amorphous materials.l'2 Mechanical treatment of zeolites and related natural and synthetic aluminosilicates showed that the changes of particulate characteristics were in many cases followed by the structural changes of treated materials, s'4 Our previous study 5 showed that ball milling of zeolites A and X and synthetic mordenite causes their transformation to the fully X-ray amorphous materials, for the long enough milling time. T h e loss o f crystallinity, the decrease o f cationexchange capacity, and the increase of solubility were caused by breaking o f "external" S i - O - S i and S i - O - AI bonds in the zeolite structure. 5 Scanningelectron micrographs of the sample obtained during the ball milling of synthetic mordenite showed that the ball milling resulted in the formation of polydispersed powder with a markedly irregular particle shape. 5 Hence, one can assume that the structural changes o f zeolites are followed by significant changes of the particulate and morphological properties during their mechanical treatment. For this Address reprint requests to Dr. Subotic, Ruder Boskovic Institute, P.O. Box 1016, Bijenicka 54, 41001 Zagreb, Croatia Received 10 February 1994 Zeolites 15:247-252, 1995 © Elsevier Science Inc. 1995 655 Avenue of the Americas, New York, NY 10010

reason our intention is to investigate the possible changes of particulate and morphological characteristics as well as the relations between them during the ball milling of zeolites A and X.

EXPERIMENTAL Samples of hydrous zeolite 4A (22.8 wt% H20 ) and zeolite 13X (26.4 wt% H20), both products of Union Carbide Corp., were milled in a planetary ball mill (Fritsch Pulverisette type 05002) at room temperature. For this purpose, 1 g of zeolite was put in an agate vessel containing 10 wolfram carbide balls (¢) = 10 mm) and the vessel was rotated (speed of rotation was 3000 rpm) for predetermined time, t,,. Depending on the duration of milling, temperature of the samples can be increased to 40-60 °. Starting zeolite powders and the samples obtained by the ball milling were characterized as follows. Content of water in the samples was determined from the c o r r e s p o n d i n g t.g. (thermogravimetry) curves. Thermal analysis of the samples were performed by a Netzch STA 409 thermal analysis apparatus. Scanning-electron micrographs of the samples were taken by a JEOL JSM-T330A scanning-electron microscope. The X-ray diffractograms of the samples were taken by Philips vertical g o n i o m e t e r PW 1820 mounted on PW 1300 X-ray generator. CuK,~ radiation was used. The weight fractions, fc of crystalline 0144-2449/95/$10.00 SSDI 0144-2449(94)00022-K

Particulate properties of zeolites during ball milling: C. Kosanovi6 et al.

and fa of amorphous phases, were calculated by the mixed method ° using the integral value of the broad amorphous halo (20 = 17-39 °) and the corresponding sharp peaks of crystalline phases. The average values of effective crystallite size were determined by the integral width of the diffraction peaks 622 (20 = 23.99°), 642 (20 = 27.12°), and 820 (2e = 29.94 °) of zeolite A and by the integral width of the diffraction peaks 533 (20 = 23.31°), 555 (20 = 30.94°), and 642 (20 = 26.65 °) of zeolite X. After subtracting the background and resolving the sharp maxima from broad amorphous halo, the profiles were corrected for instrumental broadening using the germanium diffraction profile of the 111 maximum. The effective crystallite size was calculated by using the Scherrer formula. 7 Particle size distribution curves of the samples of zeolites 4A and 14X milled for different times, tm, were determined by a Disc Centrifuge with Photosedimentometer Mark-III (Joyce-Loebl). The mean hydrodynamic particle diameter, D, geometric specific surface area, As, and specific n u m b e r of particles (particles/g), N s, were calculated from the corresponding particle size distribution curve as:

= E ~iDi/E ~i

(1)

A s = 6 E ~i (Di)2/P y" ~i (Di) s

(2)

N, = 6 E ~i/II p E ~i (Di) 3

(3)

where ~i is the number frequency of the particles having the hydrodynamic diameters between D and ~tD, D i = D + 0 / 2 and 9 is the density of the solid phase. Corresponding values of E rbi, E ~)i Oi, ~ I~)i (Oi) 2, and E r~i (Di)~ were calculated from the particle size distribution curves by the procedures described earlier. 8,9 RESULTS A N D D I S C U S S I O N Figures 1 through 4 and Tables 1 through 3 show that the ball milling of zeolites A and X causes significant changes of their particulate and morphological properties. The starting, short-term milling (tin = 10 min) caused a comminution of the original crystals of zeolite A (see Figure la) and zeolite X (see Figure 2a) and formation of two different particulate systems: damaged, but recognizable crystals of zeolite A (see Figure lb) and zeolite X (see Figure 2b) and small particles of

Figure 1 Scanning-electron micrographs of original powder of zeolite A (a) and of the samples obtained by ball milling of the zeolite A for t m = 10 min (b); t m = 20 min (c); and t m = 75 min (d).

248

Zeolites 1 5 : 2 4 7 - 2 5 2 , 1995

Particulate properties of zeolites during ball milling: C. Kosanovid et al.

Figure 2 Scanning-electron micrographs of original powder of zeolite X (a) and of the samples obtained by ball milling of the zeolite X for t,, = 10 min (b); t m = 20 min (c); and t,, = 75 min (d).

irregular shape. Because the solids obtained by the short-term milling of zeolites A and X contain about 30-40% of X-ray amorphous phase (see Figures 4 and 5 in Ref. 5), one can assume that the amorphous part of the system is represented by the small particles of irregular shape (see Figures lb and 2b). Further milling of zeolite A (tin = 20 min) causes further decrease of particle size (see Figure lb, c and Table 1) and increase of irregularity of particle shape (see Figure lb, c) geometric specific surface area, As (see Table 1), specific number of particles (see Table 1) and fraction, f~, of the amorphous phase (fa = 0.75 for tm = 20 min; see Figure 4 in Ref. 5). In contrast to the sample obtained by milling of zeolite A for t,, = 20 min which does not contain recognizable crystals of zeolite A (see Figure lc, d), damaged, but recognizable crystals of zeolite X together with an increased fraction of small particles of irregular shape can be shown in the sample obtained by milling of zeolite X for tm = 20 min (see Figure 2c). A long enough milling (>60 min) results in the formation of fully X-ray amorphous polydispersed powder with a markedly irregular particle shape (see Figures le and 2d). Figures l d and 2d (see also Figure 7b in Ref. 5) show that the larger particles

are aggregates composed of smaller ones (below 1 I~m in size). Content of water in fully amorphized samples is lower (17.5 wt% in the amorphized zeolite A and 15.5 wt% in the amorphized zeolite X) than in the original powders of zeolites A and X. The decrease of the water content can be explained by the desorption of water caused by warming of the treated materials during the milling. Although the rates of amorphization of zeolites A and X are almost the same (see Table 1 Changes in mean particle diameter, D, geometric specific surface area, As, and specific number of particles, Ns, during the milling of zeolite A (Z-A) and zeolite X (Z-X) for different

times,t m

D (~m)

As (m z g - l )

Ns ( g - l )

(min)

Z-A

Z-X

Z-A

Z-X

Z-A

Z-X

0 20 45 60 75

5.07 3.18 3.05 2.43

3.86 3.21 3.03 2.28

0.28 0.32 0.66 0.64

0.62 0.60 0.64 0.79

2.3 x 109 5.9 x 10 s 1.8 x 101° 2.5 × 10 l°

1.2 × 10 l° 1.5 × 10 l° 1.8 × 101° 3.8 x 101°

Zeolites 15:247-252,

1995

249

Particulate properties of zeo/ites during ball milling: C. Kosanovid et al.

Figures 4 and 5 in Ref. 5), change of particulate properties of zeolite X during its ball milling is rather different than the change of the particulate properties of zeolite A during its milling u n d e r the same conditions. Although the particulate properties of the samples obtained by milling of zeolite A changed monotonically during the milling (see Table 1), particulate properties changed very little during the first hour of milling of zeolite X q'a = 0.98 for t m 60 min; see Figure 5 in Ref. 5), and significant changes of the particulate properties were observed during the further milling of almost amorphous sample (see Table I). At the same time the shape of particle size distribution curves of the samples obtained by milling of zeolite X changed considerably (see Figure 4). Change in the shape of particle size distribution curves (see Figures 3 and 4) and the shifting of maximum frequency (e.g., from 2.3 Ixm for t m 0 tO 1.3 Ixm for tm = 20 min and to 1.7 p~m for t,, = 45 min, and again to 1.3 ~m for t,, = 75 min in the case of milling of zeolite X) leads to an assumption that a part of smaller particles formed by mechanical breaking of =

Z

i

2

=

C

2

o 6

4

2 00

0

Z

2b; 2

2

4

6

D (#m) Figure 3 Particle size distribution by number of original powder of zeolite A (a) and of the samples obtained by ball milling of the zeolite A for tm = 20 min (b); tm = 45 min (c); and tm = 75 min (e). No is the number percentage of particles having the size between D and D + hD.

250

2

4

6

8

Figure 4 Particle size distribution by number of original powder of zeolite X (a) and of the samples obtained by ball milling of the zeolite X for tm = 20 min (b); tm = 60 rain (c); and tm = 75 min (e). N D is the number percentage of particles having the size between D and D + AD.

c ~ , . . . . ,

o

.

D (ttm)

o

0

•

Zeolites 1 5 : 2 4 7 - 2 5 2 , 1995

original crystals tend to form aggregates (see Figures Id and 2d), as a part of the particulate system which cyclically changes its properties by a series of aggregation and deaggregation processes during the milling. Above-mentioned observations lead to an assumption that the change of particulate properties is caused by a complex process which includes mechanical fragmentation of original zeolite crystals, aggregation and deaggregation of smaller particles of crystalline and amorphous phase, comminution of the particles of fully amorphous phase, their aggregation to larger particles by the compression of material between balls and walls of vessel as well as between balls themselves, and deaggregation of such formed aggregates. Based on the same arguments it can be postulated that process of amorphization and change of particulate properties are two parallel, but more or less independent processes. Figure 5 shows X-ray diffractograms of original powder of zeolite X (diffractogram a) as well as X-ray diffractograms of the samples obtained by ball milling of the zeolite X for 20 min (b), 40 min (c), 60 min (d), and 90 min (e). In all the presented X-ray diffracto-

Particulate properties of zeolites during ball milling: C. Kosanovi~ et aL Table 2 Change in effective crystallite sizes L622, L642, and L820 which correspond to the crystal lattice planes (622), (642), and (820) of zeolite A during the milling of zeolite A for different times, tm tm (min) 0 15 30 45 60

IR d

e I

I

I

20

40

60

80

20 Figure 5 X-ray diffractograms (relative intensity I R vs. Bragg's angles 26) of original powder of zeolite X (a) and of the samples obtained by ball milling of the zeolite X for t,. = 20 rain (b); tm = 40 min (c); t m = 60 min (d); and t m = 90 min (e).

grams only the sharp diffraction peaks of zeolite X and broad diffuse diffraction maximum of the X-ray amorphous phase can be found. This means that the transformation of crystalline to amorphous phase took place in a direct way, escaping formation of any intermediate solids different from starting crystalline and resulting amorphous phase. The same change of the diffraction patterns was observed during the ball milling of zeolite A (see Figure 1 in Ref. 5). The intensities of sharp discrete peaks of crystalline phase of zeolite A (see Figure 1 in Ref. 5) and zeolite X (Figure 5 of this work) decrease and diminish without any shifting simultaneously with appearing and increasing of the separately broad diffuse maximum of the amorphous phase with the increase of the milling time, tin- According to Weeber and Bakker z this indicates that type II of the amorphization is relevant for transformation of zeolites A and X into amorphous phases during their ball milling. A little change of effective crystallite size during the amorphization of zeolites A and X (see Tables 2 and 3)just confirms the process of direct conversion of crystalline into amorphous phase and corroborates the assumption of type II of amorphization. Table 2 shows that the effective crystallite sizes, L622 and L642, which correspond to (622) and (642) crystal lattice planes of zeolite A decreased approximately linearly with the time of milling, tin, and that the effective crystallite size, L820 which corresponds to (820) crystal lattice planes of zeolite A, was almost constant during the milling.

Lsz2 (nm)

L~2 (nm)

L820 (nm)

325 325 265 265 113

267 327 229 180 161

329 268 329 231 260

Here it can be observed that the effective crystallite sizes of all measured crystal lattice planes did not change during the first 15-20 min of the milling (in this time interval about 75% of the crystalline phase has been transformed to the amorphous one). Even, the effective crystallite size of the remaining fraction of the crystalline phase (about 25%) was decreased only for 2-3 times during the further milling (see Table 2). The constancy of the effective crystallite size La20 of zeolite A leads to an assumption that the crystals of zeolite A were not broken in [820] direction. The effective crystallite sizes, L5~3, L555, and L642, which correspond to (533), (555), and (642) crystal lattice planes of zeolite X, changed only slightly during the milling so it can be said that the effective crystallite size is constant during the milling of zeolite X (see Table 3). The analysis of the change in the effective crystallite size during the milling of zeolites A and X confirms the conclusion that type II of the amorphization is relevant for the transformation of crystalline to amorphous phase during the milling and corroborates the thesis that there is no direct relation between the stage of amorphization and the change of particulate properties during the milling. Relatively large effective crystallite size in almost amorphous samples (>100 nm; see Tables 2 and 3) shows that the loss of crystallinity during the milling is not caused by the lowering of the crystal size below the X-ray diffraction limit, but rather by structural changes on the molecular level (breaking of "external" S i - O - S i and S i - O - A 1 bonds), as discussed earlier? CONCLUSION High-energy ball milling of zeolites A and X resulted in decrease of crystallinity followed by decrease of mean particle size and simultaneous increase of the Table 3 Change in effective crystallite sizes L ~ , Lss5, and Ls42 which correspond to the crystal lattice planes (533), (555), and (642) of zeolite X during the milling of zeolite X for different times, t m tm(min) 0 20 40 60 90

Ls3a(nm)

Lsss(nm)

Ls42(nm)

217 217 202 282 217

240 207 239 261 207

237 218 142 149 237

Zeolites 15:247-252, 1995

251

Particulate properties of zeo/ites during ball milling: C. Kosanovid et al.

number of particles and their geometric specific surface area. A part of smaller particles formed by mechanical breaking of original crystals of zeolites A and X tend to form aggregates as a part of particulate system which cyclically changes its properties by a series of aggregation and deaggregation processes during the milling. Although the rates of amorphization of zeolites A and X are almost the same, change of particulate properties of zeolite X during its ball milling is rather different than the change of particulate properties of zeolite A during its milling under the same conditions. Because there is no a reasonable correlation between change of fraction of amorphous phase and the change of particulate properties during the milling, it was concluded that process of amorphization and change of particulate properties are two parallel, but more or less independent processes. Analysis of the change of effective crystallite size during the milling showed that transformation of crystalline to amorphous phase is caused by type II of the amorphization process in accordance with the classification of Weeber and Bakker, 2 and that the amorphization is c a u s e d by b r e a k i n g o f " e x t e r n a l " S i - O - Si and S i - O - AI bonds of zeolite structure rather than by the lowering of the crystal size below the X-ray detection limit.

252

Zeolites 15:247-252, 1995

ACKNOWLEDGMENT The authors thank the Ministry of Science and Technology of the Republic of Croatia for its financial support.

REFERENCES 1 Artz, E., and Schultz, L., Eds. New Materials by Mechanical Alloying Techniques, DGM e.V. Informations Gesellschaft Verlag, Oberursel, Germany, 1989 2 Weber, A.W., and Bakker, H. Physica B 1988, 153, 93 3 Basaldella, E.l., Kikot, A., and Pereira, E. React. Solids 1990, 8, 169

4 Pilipenko, A.T., Kornilovich, B.Yu., Zaplol'skii, A.K., Vasil'ev, 5 6 7 8 9

N.G., and Paschenko, E.A. Dokl. Akad. Nauk Ukr. SSR, Ser. B: Geol. Khim. Biol. Nauki 1987, 5210 KosanoviE, C., Bronid, J., Subotid, B., ~mit, I., Stubi~ar, M., Tonejc, A., and Yamamoto, T. Zeolites 1993, 13, 261 Zevin, L.S., and Zavyalova, L.L. Kolichestvenniy Rentgenographicsheskiy Prazoviy Analiz, Nedra, Moscow, 1974, p. 37 Klug, H.P., and Alexander, L.E. X-ray Diffraction Procedures, John Wiley and Sons, New York, 1954, p. 491 Suboti6, B., Ma,~id, N., and ~mit, I. in Studies of Surface Science and Catalysis, (Eds. B. Dr~aj, S. Ho~,evar, and S. Pejovnik) Elsevier, Amsterdam, Vol. 24, 1985, p. 207 Kolar, Z.I., Binsma, J.J.M., and Suboti~, B. J. Cryst. Growth 1992, 116, 473